This invention relates to a solids-free solution for introduction into wellbores and a method fof use therefor.
Subterranean formations may contain valuable mineral deposits such as hydrocarbon oils and gases, sulfur and other valuable natural resources. Contact may be made with these formations by drilling a wellbore from the surface to the subterranean formation. During the course of this drilling operation a fluid is introduced into the wellbore to lubricate and cool the drilling bit, to carry away material removed from subterranean formations by the drilling process, and to seal the walls of the borehole against loss of the fluid or invasion of the borehole by fluids in subterranean formations. Such a fluid or drilling mud is described, for example, in U.S. Pat. No. 2,073,413 which is herein incorporated by reference.
These drilling muds achieve the high density which is necessary to avoid intrusion of high pressure subterranean fluids into the borehole by weighting the mud with solid materials such as clays or barytes. High densities in the order of magnitude of 18-20 lb/gal are achievable with such drilling muds.
However, at certain times during the well drilling and completion process, it is desirable to have solids-free solutions contacting the subterranean formations in the borehole. For example, see the article by J. L. Kennedy, The Oil and Gas Journal, (Aug. 2, 1971) pp. 62-64, which is hereby incorporated by reference. Many procedures require a solids-free fluid. For example, packing and completion procedures can be injured by solids suspended in the fluid within the wellbore. See Drilling and Production Practice, C. M. Hudgins et al. (American Petroleum Institute) 1961; Hudgins et al., The Oil and Gas Journal, (July 24, 1961) pp. 91-96; and J. H. Plonka, World Oil (April, 1972) pp. 88-89; Neal Adams, The Oil & Gas Journal (Nov. 9, 1981), pp. 254-275. These articles, which are hereby incorporated by reference, describe the use of high density, solids-free brines as fluids. Sodium chloride can be used to make brines from 8.33 to 9.8 lb/gas. Calcium chloride can be used to make brines from 8.3 to 11.5 lb/gal. Calcium chloride/zinc chloride brines can be used to make brines from 11.5 to 14.0 lb/gal. The Hudgins articles describe zinc-containing fluids with densities greater than 14.0 lb/gal as being too corrosive for practical use. The Plonka article describes a solids-free calcium bromide/calcium chloride solution which can achieve a density of 15.0 lb/gal.
U.S. Pat. No. 4,292,183 describes a zinc bromide/calcium bromide solids-free solution having a density lying in the range from about 14.5 up to about 18.0 lb/gal. The Adams article describes a CaCl.sub.2 /CaBr.sub.2 /ZnBr.sub.2 brine system with densities up to 19.2 lb/gal, as well as characteristics of the various fluids and methods of use.
It is sometimes desirable to have a solidsfree solution which has a density of greater than 15.0 lb/gal. Until this invention, this required the use of a zinc-containing fluid.
A desirable well-servicing fluid needs to have several characteristics concurrently. The density of the fluid should be such that the hydrostatic head formed by the column of fluid in the wellbore properly balances the fluid pressure of the subterranean formation. Appropriate balance may at times be slightly below the pressure of the subterranean formation, is often greater than the pressure of the subterranean formation, but most often is set to exactly match the hydraulic pressure of the fluid in the subterranean formation.
Fluids used during workover and completion range from low-density gases, such as nitrogen, to high-density muds and packer fluids. The applications and requirements for each fluid are different.
Fluids used during the reworking of a well after its initial completion are termed workover fluids. These fluids may be gases (such as nitrogen or natural gas), brine waters, or muds. The functions performed by the workover fluid include well killing, cleaning out a well by removing sand, rock, or metal cuttings, and other foreign objects, drilling into a new productive interval, or plugging back to complete a shallower interval.
Completion fluids are used during the operations that establish final communications between the productive formation and the well bore. The fluids may be a commericial workover fluid, nitrogen, or a clean, low-solids brine water and may be used for a short period of time such as well perforating or for extended periods such as in gravel packing. The primary requirement placed on the fluid is that it does not damage nor block the producing formation.
Packer fluids are placed in the annulus between the production tubing and casing. The most common requirements for packer fluids are to maintain pressure control, be nontoxic and noncorrosive, remain pumpable, and minimize formation damage.
The well-servicing fluid should be solidsfree. That means substantially free from suspended solids of greater than about 5 microns in diameter. It also means that the solution should have a crystallization point lower than the use temperature. Generally, the crystallization point is the minimum temperature at which the soluble solids are dissolved to form a solidsfree solution.
Corrosivity is also an important factor. The corrosivity of the brine solution should be such that no significant corrosion of metal piping or drilling implements occurs. This generally requires an uninhibited corrosion rate of less than 0.005 inch/year. This is particularly true when the fluid is used to shut in a well. The fluid may be in contact with the downhole piping for long periods of time. It is also important to minimize harm to the environment.
When one requires a solution with a density of greater than 15.0 lb/gal to balance the formation pressure, each of the previous recited fluids has shortcomings. All of the brines except the zinc-containing brines will not form a solids-free solution at greater than 15.0 lb/gal with a crystallization point of less than 20.degree. C. The densities and solubilities of the solid salts will not permit it. While the zinc-containing brines may form solids-free solutions at greater than 15.0 lb/gal, these solutions are generally more corrosive and may harm the environment. It is therefore an object of this invention to provide a solids-free well-servicing fluid with a density of greater than 15.0 lb/gal and a crystallization point of less than 20.degree. C. that does not require the use of zinc salts.